This invention relates to the field of solid state photo-sensors and imagers referred to as Active Pixel Sensors, (APS) and more specifically to providing a means to provide programmable color balance for a color image sensor, while maintaining a high signal to noise ratio (SNR).
APS devices are solid state imagers wherein each pixel typically contains a photo-sensing means, reset means, a charge transfer means, a charge to voltage conversion means, and all or part of an amplifier. APS devices have been operated in a manner where each line or row within the imager is selected and read out using a row and column select signal, (analogous to a word and bit line in memory devices respectively).
Typical prior art APS pixels are shown in FIGS. 1a and 1b. The pixel in FIG. 1a comprises a photodetector (PDET), that can be either a photodiode (PD) or photogate (PG), transfer gate (TG), floating diffusion (FD), reset transistor with a reset gate (RG), row select transistor with a row select gate, (RSG), and signal transistor (SIG). The pixel in FIG. 1b comprises a photodetector (PDET), typically a photodiode (PD), reset transistor with a reset gate (RG), row select transistor with a row select gate, (RSG), and signal transistor (SIG). In all prior art pixels the TG, RG and RSG nodes within a pixel were connected together by a single buss per row of pixels to facilitate the per row sensor readout mode described above. Since the integration time for the pixel in FIG. 1a is determined by the time from when TG is turned off to begin integration and then turned on to transfer charge to the FD, the integration time for each pixel in that row was the same. This is also the case for the pixel in FIG. 1b, where the integration time is determined from the time that RG is turned on to reset the PD and when the read signal is applied to read the charge in the PD.
In prior art devices, the image capture for each row is typically done sequentially with the image capture for each row temporally displaced from every other row, and each row has the same integration time. In color imaging applications a color filter array (CFA) is placed over the image sensor. These individual color filters typically, allow varying amounts of optical energy to pass through the filter and become incident on the sensor. Color filters do not inherently pass the requisite amount of optical energy desired for color balance or that desired for display purposes.
Accordingly, signal processing, whether analog or digital, must be performed on the sensor output to provide the appropriate gain to each color channel in order to produce the desired color balance. With APS devices, this voltage domain signal processing can be done on chip, but this results in the disadvantages of: (1) inducing noise in the sensor; and (2) requiring complicated circuits that consume silicon area, dissipate more power, and slow down frame rates.
In view of the foregoing discussion, it should be apparent that there remains a need within the prior art for an APS device with improved color balancing.
The present invention addresses the previously discussed problems by enabling programmable color balancing of a color image sensor without creating the disadvantages described above. One approach to providing a programmable color balance means without voltage domain signal processing is to adjust the integration time for each color. In this manner, the resulting signal charge generated within the pixel is in proportion to the desired color balance and no other post processing is required. This invention provides a manner of separate integration control for any given color while maintaining the same sensor readout scheme and allowing for the elimination of the added voltage domain color balance signal processing required in prior art devices. This is done by providing separate transfer gate busses or reset gate busses for each color in a given row. By doing so, the integration time for each color within a given row can be controlled separately.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, presents a semiconductor based image sensor comprising: a plurality of pixels formed upon a major surface of a semiconductor substrate, each of the pixels having a photodetector; a plurality of color filters of at least two different colors arranged over the pixels; a plurality of transfer gates, the transfer gates being adjacent to the photodetectors and controlled by one of a plurality of electrodes, the transfer gates being capable of modulating the electrostatic potential adjacent to the photodetector upon application of a predetermined voltage to the electrode for that transfer gate to transfer charge from the photodetector to a sense node; a plurality of busses arranged such that there is at least one color buss for each color of the different colors, each of the busses being operatively connected to the transfer gates associated with only one of the different colors through one of the electrodes; a timing circuit having at least one connection to each of the color busses and capable of generating the predetermined voltage on the busses at a predetermined time; whereby each of the different colors can have an integration period that is controlled by the timing circuit independent other of the different colors.
These and other aspects, objects, features, and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
Advantageous Effect of the Invention
The present invention has the following advantages: it is efficient in the use of silicon area, it provides a high Signal to Noise Ratio (SNR), and it provides programmable color balance.